1. Field of the Invention
This invention relates to a limited slip differential and, more particularly, to a limited slip differential suitable to be installed on a drive line of an automobile.
2. Description of the Prior Art
A conventional limited slip differential, in which a plurality of pinions consisting of straight bevel gears and a pair of side gears consisting of straight bevel gears meshing with these pinions are disposed in a differential case, has frictional force generating means which limits the differential motion produced between shafts coupled respectively with the side gears. In the limited slip differential, a multiple disc clutch is pressed with the axial thrust load produced in the side gears to obtain the frictional force. However, this frictional force has the same magnitude either at the time of driving and at the time of being driven such as engine braking. As a result, the differential limiting forces generated at driving and driven times are of the same magnitude.
It is an original object of a limited slip differential to effectively transmit power to drive wheels at the time of driving (driving forwards). From this point of view, a large differential limiting force is preferably generated at the time of driving while a small differential limiting force is preferably generated at the time of being driven. Particularly, in a vehicle incorporating an anti-lock brake system or like apparatus for preventing wheel lock at the time of braking, a difference in braking force between left and right wheels occurs when the large differential limiting force acts at the time of engine braking or braking operation. As a result, yawing moment is generated about a vertical axis extending through the center of gravity of the vehicle to deteriorate the braking stability.
Japanese Patent Public Disclosure (KOKAI) No. 61-41038 has disclosed a differential gear in which a pair of sun gears or side gears and a plurality of planetary gears or pinions meshing with the side gears respectively disposed in a differential case are constituted from spiral bevel gears having respective teeth twisted in the opposite direction. In this differential gear, the tooth surfaces on which the pinions mesh with left and right side gears are concave with respect to the side gear on one side and are convex with respect to the side gear on the other side. Accordingly, the axial thrust may be varied to enable enlargement of the differential limiting force at the time of driving.